Hello. Thank you for contacting the Toolset support.
Well - as I understand, you need dependent dropdown so that if you select one option from the first dropdown it will display the associated options with the second drop-down. If this is true - there is no such native option available using which you can construct dependent drop-down using parent/child taxonomy.
There is a way using which you can construct/dispaly dependent dropdown using views but it works based on the post type. You can check the example at:
=> hidden link
[ check the "State" and "City" dropdowns]